Fullerene Nanotubes: C1,000,000 and Beyond
Some unusual new molecules—long, hollow fibers with tantalizing electronic and mechanical properties—have joined diamonds and graphite in the carbon family
Originally published in the July-August 1997 issue of
Imagine holding in your hand a wand that is a single molecule. Such
a wand would be so thin that it could not be seen under an ordinary
microscope; it would be a nanotube, just a few atoms in
circumference. But it could be very long. Such a tubule in fact
exists in the laboratory today, and it has even been used there as a
probe, poking down into the world of molecules. Called the carbon
nanotube, it is elongated like a fiber, and yet it is hollow and
inherits the perfection of atomic arrangement made famous by its
predecessor, the buckyball—the remarkable closed cage of 60
symmetrically arranged carbon atoms that was recognized as a new
form of carbon when it was discovered a decade ago. The buckyball's
molecular family, the fullerenes, has expanded since that discovery;
the nanotube is a new and very useful addition.
Carbon nanotubes are, effectively, buckyball structures played out
as long strands rather than spheres. Their length can be millions of
times greater than their tiny diameter. Their properties as a new
material are remarkable—a fact that was evident almost as soon
as they were first spotted in 1991, turning up in the soot and dirt
piles that fill chambers where scientists produce fullerenes, large
geometric carbon molecules.
A chemist might think of a carbon nanotube as a monoelemental
polymer. (Most polymers, such as polyethylene, are carbon chains
with other elements attached.) The buckyball is designated
C60; a carbon nanotube might be C1,000,000. In
physics terms it can be described as a single crystal in one
direction, with a unit cell that keeps on propagating and repeating.
This periodic pattern has the symmetry of a helix, not as complex as
the double helix responsible for life itself, but possessing a
special beauty in its monotonous order—a molecular incarnation
of Ravel's Bolero. A mathematician will be delighted by the
symmetry and rigor of these structures and how nicely they obey
Euler's rule for polyhedra.
Likewise the nanotube has what a civil engineer would recognize as a
beam-and-truss construction, a billion times smaller than such
structures built to human scale. To satisfy the standard chemical
requirements of carbon, every beam ties two carbon atoms by a strong
covalent bond, and every atom accommodates exactly three neighbors.
A nanotube, no matter how long, must thus ultimately be sealed on
the ends to leave no dangling "unhappy" chemical bonds.
The strength of these bonds and their clever organization makes
nanotubes so highly resistant to tension that (with apologies to
high-energy physicists) they could justly be called superstrings.
Furthermore, electrons move easily along some carbon tubules,
although their minuscule cross section permits electrical
conductance only in a quantized fashion.
Several analogies, some of them famous, have been invented by
science teachers to help their students and themselves comprehend
the smallness of the small, and these can help us imagine nanotubes.
One difficult dimension to imagine is the nanometer, a billionth of
a meter. One could stare at a chip of graphite as Richard Feynman
once stared at a drop of water. After magnifying our chip a billion
times, creating a metal-gray rocky landscape about the size of
Texas, we might spot a three-foot-diameter pipeline stretching from
horizon to horizon. This is the nanotube. Actually, a
one-nanometer-wide pipeline occupies almost no space even over a
substantial length. In fact, nanotubes sufficient to span the
250,000 miles between the earth to the moon at perigee could be
loosely rolled into a ball the size of a poppyseed. Together, the
smallness of the nanotubes and the chemical properties of carbon
atoms packed along their walls in a honeycomb pattern are
responsible for their fascinating and useful qualities.